Laser coding is well known; see, for example, U.S. Pat. No. 5,733,793, U.S. Pat. No. 4,906,813 and also U.S. Pat. No. 5,340,628 which seeks to contain the particles produced by ablation. These methods present a variety of problems, including difficulties in maintenance, line down-time, taint, as well as the need for extraction. More generally, the apparatus and problems of printing, i.e. ribbons, inks, solvents, maintenance, unreliability, etc., are particularly undesirable where sensitive products like foods and pharmaceuticals are packaged.
Various proposals have been made, in order to achieve effective printing without ablation, and without applying ink at the point of coding, but rather by causing a change of colour in the substrate on which the printing is to appear. Various pigments have been proposed, which can be used to mark a substrate on the application of laser energy. Some of these proposals may be found in, for example, WO-A-00/43456, JP-A-11001065, EP-A-0522370, EP-A-0797511, U.S. Pat. No. 5,053,440, U.S. Pat. No. 5,350,792 (a plastic moulding composition comprising a polyoxymethylene and animal charcoal), U.S. Pat. No. 5,928,780, U.S. Pat. No. 6,017,972 and U.S. Pat. No. 6,019,831.
On-line coding methods commonly used for the pharmaceutical, foods and confectionery industries are inkjet and thermal transfer (including hot stamping).